Acoustic well logging tools are devices that include a vibrator which forms an acoustic wave front in drilling fluid. The drilling fluid, normally known as drilling mud, fills a well borehole where the entire length of the well encloses a column of standing liquid which prevents blowouts. That is, the drilling fluid prevents gaseous petroleum products from being produced and creating an excessive flow before the well can be brought under control. Moreover, drilling fluid is necessary in a well borehole to provide lubrication of the drill bit and to also remove cuttings made by the drill bit during drilling operations. An acoustic transmitter is normally used in a well to perform a test. One purpose of acoustic transmitters is to measure the quality of the bond between the casing which surrounds the well borehole and the surrounding cement layer which is positioned on the exterior of the casing. Moreover, the acoustic device forms a pulse which ideally is varied in frequency to provide maximum adaptability for the acoustic test device. Normally, this is placed in a sonde which has a housing for equipment adapted to be lowered on a well borehole on a logging cable and which is connected to the surface through one or more conductors in the logging cable. This enables signals to be transmitted up the logging cable to located equipment surface. The signals transmitted to the surface are used at the surface to provide data for the quality of the cement bond or other information regarding the partially completed well.
It is necessary to vary the amplitude and the frequency of an acoustic signal. Acoustic transmitters used heretofore have varied widely and have included devices utilizing piezoelectric transducers, electromagnetic transducers or magnetostrictive transducers. Additionally, hydraulically powered transducers have also been used. The amount of power that a transmitter can produce is fairly well defined by the stroke of the mechanism which drives the acoustic transmitter. In addition, the amount of power is also dependent on the cross-sectional area of the device which provides the pulse transmission. The active device can be enlarged to increase its area and thereby increase the power in the stroke. If possible, the area can be expanded by using various magnification devices. This has its own limitations, those being noted below. One way of increasing power is to enhance the piston stroke. This can be done by connecting the driven item with a powered piston through some kind of mechanical linkage or leverage device. As a generalization, this will enhance the stroke but it is accomplished at the cost of increasing the weight of the moving components. If the weight were doubled, the inertial drag on the system would require twice the power. What is desired is a low mass system to reduce the reactionary force resulting from inertia. Otherwise, the inertial drag would become so great that the stroke is reduced when the moving surface or area is operated.
The present disclosure is directed to a large surface area transmitter which is hydraulically powered. It incorporates a relatively light mass, relatively large exposed diaphragm of modest thickness. In one embodiment, it is exposed at both planar faces. The planar faces are preferably exposed so that a positive signal can be transmitted from one face while a negative signal is transmitted from the opposite face. This provides a compression wave on one side while simultaneously forming a rarefaction wave on the opposite side. By doing this, a very large area can be operated. The large area diaphragm which makes up part of the present structure is vibrated by connecting it at two spaced end portions to driver pistons which connect with drive shafts which vibrate the diaphragm. The two pistons are identical in construction and are deployed on opposite sides of the diaphragm. The diaphragm in the preferred embodiment is rectangular to obtain the maximum area. It fits snuggly in the tool housing but it need not be sealed to it. The two hydraulically powered drivers incorporate protruding piston rods (or drive shafts) connected with appropriate pistons. The pistons are received in respective hydraulic cylinders which permit the pistons to be operated in a vibration forming mode of use. More will be noted concerning the hydraulic system which operates the two pistons. Preferably they are operated in a synchronized fashion, namely they operate at the same frequency and are 180.degree. out of phase. Restated, they extend simultaneously and retract simultaneously. This creates a flexure which travels through the flexible diaphragm, forming the propagated wave. Generally speaking, the diaphragm or membrane is curved in the relaxed state. This actually increases the incremental travel distance. Thus, if the ends are deflected only a fraction of an inch, the central portion of the curved diaphragm will travel a much greater distance and thereby form an amplified outwardly radiated signal. This signal is formed from the acoustic diaphragm which functions as a dipole source. The ratio of the displacement of the center of the diaphragm with respect to the length of travel is the effective displacement amplification factor. This is relatively high. Simultaneously the inertial drag on the system is relatively low because the only moving component is the diaphragm and the mud immediately against it. While the two hydraulic pistons move axially, they move in opposite directions to each other to reduce the loads that are inflicted on the supporting tool. One aspect of the present apparatus is that it can be used to form a compression wave which is directed radially outwardly and also form a shear wave moving along the length of the well borehole in which the tool is installed. More particularly, the shear wave is formed so it propagates along the well borehole. The transmitted acoustic waves in the drilling fluid which fills the well enable the acoustic tool to operate in the intended fashion with an appropriate acoustic receiver. It is believed that acoustic receivers of the sort cooperative with this disclosure are well known.